PURPOSE: To reduce the capacity and to make small the unit, by giving the gate pulse of a given width only when the actual trigger operation is required for the semiconductor circuit breaker.

CONSTITUTION: When the load 11 is at no-load state, the load impedance viewed from the power source becomes very great. On the other hand, since the impedance of the semiconductor circuit breaker 12 can be neglected, the power source voltage cannot almost be fed to the semiconductor circuit breaker. Next, with load, the state of the said impedance is inverted, the power source voltage is almost given to the semiconductor circuit breaker 12 and the forward voltage appears. Accordingly, the forward detector 13 is applied between the poles of the circuit breaker 12, and with the logical product signal with the forward voltage detection signal 15 and the on-command signal 16 of circuit breaker, the gate pulse 17 with preset width is produced, the trigger gate signal is fed to the semiconductor circuit breaker 12 to turn on the semiconductor circuit breaker 12.